OCR Text |
Show For an extended plane surface it depends above all on the wind velocity, u, and the surface roughness. The following table gives some typical values: u 1 2 5 ms- i 1. * L 2,5 4,4 9,5 meal cm-* min* 1 grd- 1 2. « x 8 12 19 meal cm- 8 min- 1 grd- 1 3. at, 10 14 22 meal cm-* min" 1 grd- 1 When u is understood as the wind velocity at 2 m, the values opposite 1. are valid for the layer 0- 2 m and for a completely smooth surface. The values opposite 2. are from measurements by M. de Quervain T5] over a snow surface, those opposite 3. are determined from a formula given by E. Frankenberger [ l] for a grass surface. As a rough approximation for CC^_ , the value CC^ = 10 meal cm"^ min"*- deg" 1 can be taken, which agrees with H. G. MUller's [ 4] estimate of 11.2 meal cm"^ min~ l deg"*-. Values of the heat transfer coefficient, ( X. , in the above table are mean values for extended surfaces, but variations from place to place obviously can occur. Especially high values occur at ridges and points, where CC. can easily reach ten to a hundred times the values listed above, while at other sites CC^ has lower values. The latter, for example, is true of depressions in the ground, for here the air layer with reduced or even molecular heat transfer is the thickest. In the middle of an extended surface without too great an unevenness, these differences balance one another so that the mean ablation applies. Over a rough surface the mean heat transfer coefficient of course is greater than over a smooth one, as the table shows. At the individual surface elements with different heat transfer coefficients, the ablation - M in general varies, and does so even when the air temperature > V7 , 10 |